The present invention generally relates to electronic filters and, more particularly, to such filters that include tunable dielectric capacitors (dielectric varactors).
One of most dramatic developing areas in communications over the past decade has been mobile and portable communications. This has led to continual reductions in the size of the terminal equipment such as the handset phone. Size reduction of the electronic circuits is progressing with the development of recent semiconductor technologies. However, microwave filters occupy a large volume in communications circuits, especially in multi-band applications. Multi-band applications typically use fixed filters to cover different frequency bands, with switches to select among the filters. Therefore, compact, high performance tunable filters are extremely desirable for these applications, to reduce the number of filters and simplify the control circuits.
Electrically tunable filters are suitable for mobile and portable communication applications, compared to other tunable filters such as mechanically and magnetically tunable filters. Both mechanically and magnetically tunable filters are relatively large in size and heavy in weight. Electronically tunable filters have the important advantages of small size, lightweight, low power consumption, simple control circuits, and fast tuning capability. Electronically tunable filters can be divided into two types: one is tuned by tunable dielectric capacitors (dielectric varactors), and the other is tuned by semiconductor diode varactors. The dielectric varactor is a voltage tunable capacitor in which the dielectric constant of a dielectric material in the capacitor can be changed by a voltage applied thereto. Compared to semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed. Third intermodulation distortion happens when two close frequency signals (f1 and f2) are input into a filter. The two signals generate two related signals at frequencies of 2f2−f1 (say f3), and 2f1−f2 (say f4), in addition to the two main signals f1 and f2. F3 and f4 should be as low as possible compared to f1 and f2. The relationship between f1, f2, f3 and f4 is characterized by IP3. The higher the IP3 value is, the lower the third intermodulation. Considering the additional attributes of low power consumption, low cost, variable structures, and compatibility to integrated circuit processing, dielectric varactors are suitable for tunable filters in mobile and portable communication applications.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled “Ceramic Ferroelectric Material”; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO—MgO”; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO—ZrO2”; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-Magnesium Based Compound”; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; and U.S. Pat. No. 5,635,433 to Sengupta, entitled “Ceramic Ferroelectric Composite Material—BSTO—ZnO”. These patents are hereby incorporated by reference. A copending, commonly assigned U.S. patent application Ser. No. 09/594,837, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTO—MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Commonly used compact fixed filters in mobile and portable communications are ceramic filters, combline filters, and LC-lumped filters. This invention provides tunable filters, utilizing advanced dielectric varactors.